2003 toyota 4runner tail light pin out?

I am trying to tap directly into the wire harness that runs into the tail light provide power for my backup camera. It has 5 wire. So I turned on my car and put it in reverse... and check it with a multimeter... So I figure out that the red wire is the pos for the backup light. Now... the interesting thing is that the rest of the wire seem to be grounded when the other light are off and seem to be switched on depend on what light is on. So now my concern is that which wire is really the ground for the back up light. I don't know if I am making any sense but it seem like the whole vehicle is grounded together when it's turn off... and depend on whatever got switch on then that wire become "live" but as soon as it's off the wire seem to be grounded. If anybody can shred any light into this I would really appreciate it. I am not sure what would be the best place for this post... so if it's in the wrong area please let me know and I'll move it.